Nanocrystalline

Nanocrystalline materials are solids in which a substantial fraction of the crystalline grains occur at nanometer scales. Most definitions cite average grain sizes below 100 nanometers, though some reports include up to a few tens of nanometers. The term is used for metals, ceramics, and composites that possess crystalline nanostructures rather than amorphous structures.

In nanocrystalline materials, a large fraction of atoms resides in grain boundaries, which strongly influence properties. They can be produced by top-down approaches such as severe plastic deformation (high-energy ball milling, equal-channel angular pressing) and by bottom-up methods such as chemical synthesis, vapor deposition, and rapid solidification. Consolidation techniques like spark plasma sintering can produce dense nanocrystalline ceramics and metals.

Mechanical properties are dominated by the high volume of grain boundaries. Nanocrystalline metals often show increased strength and hardness due to grain-boundary strengthening, described by the Hall-Petch relation. At the smallest grain sizes, some materials experience softening or reduced ductility, related to grain-boundary sliding and limited dislocation activity. Thermal stability is a concern; grains can coarsen at elevated temperatures, diminishing hardness unless stabilized by alloying or dopants.

Characterization commonly uses transmission electron microscopy and X-ray diffraction (Scherrer analysis) to estimate grain sizes, along with electron backscatter diffraction for grain-boundary statistics.

Applications include wear-resistant coatings and structural components for metals, and catalysts, sensors, and advanced ceramics where high strength and large grain-boundary areas are advantageous.